This invention was partially funded by National Natural Science Foundation of China grant nos. 61205024 and 61178038, and Hi-Tech Research and Development Program of China grant no. 2009AA01Z308.
The present invention relates to a head-mounted display device, and in particular, to a tiled head-mounted display device comprising wedge-shaped prisms with free-form surfaces.
Head-mounted display devices for virtual environment and augmented environment are popular products in display industry and have been extensively developed in recent years. Head-mounted display device can be used in consumer applications such as 3D movie, video game and sports as well as high end applications such as scientific research, medical/industry training, flight simulation, immersed entertainments. To be a useful and valid display system, the head-mounted display must be capable of generating high fidelity and wide field of view scene. The compactness and lightweight are also preferred to reduce user's neck fatigue.
A head-mounted display device typically consists of three parts: display component, optical system and helmet. In order to reduce the weight of the head-mounted display device, it is crucial to use an optical system with a short focal length and a micro-display. However, there is a trade-off between compactness of optical system and imaging quality of head-mounted display. For head-mounted display device, it is necessary for the optical system to have a large field of view and large exit pupil diameter. The large field of view increases the sense of immersion and allows the users to observe mobile object better. The large exit pupil diameter allows the users to arbitrarily move their eyes during observation without image lost. It would also allow various users with different interpupillary distance to use the system without adjusting the interpupillary distance of the helmet. However, it is difficult to achieve wide field of view, large exit pupil diameter and high resolution at the same time, due to the tradeoff relationships among these parameters.
For a conventional head-mounted display system employing a single display channel with a single micro-display for each eye, the relationship between the field of view and the resolution may satisfy R=N/FOV, where R is resolution of the display system, N is resolution of the single micro-display, and FOV is a field of view of the display system. R and FOV are mutually restricted by each other with a given N value, that is, a large field of view will result in a low resolution. Therefore it is difficult to satisfy the requirements of large field of view and high resolution simultaneously in a conventional head-mounted display device employing a single display channel.
A tiled head-mounted optical display system based on conventional rotational symmetry oculars is proposed in J. E. Melzer's paper titled “Overcoming the field-of-view/resolution invariant in head-mounted displays”, Proc. SPIE, Vol. 3362, 284 (1998), L. G. Brown's paper titled “Applications of the Sensics panoramic HMD ”, SID Symposium Digest 39, 77 (2008), and M. Gutin' paper titled “Automated design and fabrication of ocular optics”, Proc. SPIE 7060, (2008). FIG. 1a shows a schematic view of a tiled optical system and a schematic view showing distortion correction of each display channel of the system. FIG. 1b shows a schematic view of image shown on a screen observed through the system formed by tiling two oculars with rotational symmetry when the micro-displays in the system display an image of regular rectangles.
As shown in FIG. 1a, the tiled head-mounted optical display system based on the conventional rotational symmetry oculars requires a great number of oculars tiled together in order to obtain a satisfied field of view. The rotational deviation of the respective display channels and the corresponding micro-displays from the user's viewing axis leads to tilting in the image planes of display channels. In this case, image magnification varies throughout the tiled system, resulting in image distortion on the pupil plane for the displays located at the edges. As shown in FIG. 1b, a regular rectangle image displayed by micro-displays is observed as a trapezoid through the rotational tiled oculars. Therefore, the images to be displayed on each display of the tiled optical system needs to be pre-warped, otherwise the user will observe distorted images. For example, the warping for the regular rectangle image is shown in the right portion of FIG. 1a. The image displayed on centeral displays is unchanged and is still a regular rectangle, while the image displayed on marginal displays needs to be pre-warped to be as a trapezoid.
In addition, in tiling process, the oculars at the edges need to be rotated around the center of the exit pupil of the system, therefore, the eye clearance, that is, the minimum distance from human eyes to the tiled oculars is reduced. As shown in FIG. 1a, the eye clearance (ec′) of the optical tiled system is less than the eye clearance (ec) of an ocular with a single display channel. Therefore, in order to satisfy the overall requirements of the eye clearance, so that such system can also be used by the users who wear for example glasses or mask, the exit pupil distance or eye clearance of a single ocular must be increased. Moreover, the exit pupil planes of the respective oculars do not coincide with each other but are tilted relative to each other. Therefore, users may see discontinuous images when their eyeballs are moving. The decrease of the effective exit pupil diameter may also lead to pupil aberration. In addition, in the system shown in FIG. 1a, the ocular with rotational symmetry is located between the user's eyes and the micro-displays. If the head-mounted display device is used for augmented environment, a transmissive-reflective optical component needs to be added in order to satisfy the requirements of optical transmission and reflection. In that case, the ocular size will be further increased in order to ensure minimum eye clearance (from half mirror to user's eyes). For a system tiled by a plurality of oculars, the structure of optical system is greatly complicated, and the weight and size of optical systems are increased significantly.
Moreover, the tiling process of the oculars with rotational symmetry is complex, requiring additional processes for the tiling surfaces. For this tiled system, positions and angles of the tiled surfaces of each ocular are different depending on the positions of the oculars in the tiled system. For the ocular at the center, it is necessary to process three or four tilted tiling surface. The processing requirements for oculars at different position are also different. Therefore, it is very difficult to process and assemble this tiled system with relatively high precision.
Therefore, there is a demand for a new kind of head-mounted display device having a large field of view and a high-resolution.